LM139AQML, LM139QML
SNOSAH8G –FEBRUARY 2005–REVISED MARCH 2013
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APPLICATION HINTS
The LM139 series are high gain, wide bandwidth devices which, like most comparators, can easily oscillate if the
output lead is inadvertently allowed to capacitively couple to the inputs via stray capacitance. This shows up only
during the output voltage transition intervals as the comparator changes states. Power supply bypassing is not
required to solve this problem. Standard PC board layout is helpful as it reduces stray input-output coupling.
Reducing this input resistors to < 10 kΩ reduces the feedback signal levels and finally, adding even a small
amount (1 to 10 mV) of positive feedback (hysteresis) causes such a rapid transition that oscillations due to stray
feedback are not possible. Simply socketing the IC and attaching resistors to the pins will cause input-output
oscillations during the small transition intervals unless hysteresis is used. If the input signal is a pulse waveform,
with relatively fast rise and fall times, hysteresis is not required.
All pins of any unused comparators should be tied to the negative supply.
The bias network of the LM139 series establishes a drain current which is independent of the magnitude of the
power supply voltage over the range of from 2 V
DC
to 30 V
DC
.
It is usually unnecessary to use a bypass capacitor across the power supply line.
The differential input voltage may be larger than V
+
without damaging the device. Protection should be provided
to prevent the input voltages from going negative more than −0.3 V
DC
(at 25°C). An input clamp diode can be
used as shown in the Typical Applications section.
The output of the LM139 series is the uncommitted collector of a grounded-emitter NPN output transistor. Many
collectors can be tied together to provide an output OR'ing function. An output pull-up resistor can be connected
to any available power supply voltage within the permitted supply voltage range and there is no restriction on this
voltage due to the magnitude of the voltage which is applied to the V
+
terminal of the LM139A package. The
output can also be used as a simple SPST switch to ground (when a pull-up resistor is not used). The amount of
current which the output device can sink is limited by the drive available (which is independent of V
+
) and the β
of this device. When the maximum current limit is reached (approximately 16 mA), the output transistor will come
out of saturation and the output voltage will rise very rapidly. The output saturation voltage is limited by the
approximately 60Ω R
SAT
of the output transistor. The low offset voltage of the output transistor (1 mV) allows the
output to clamp essentially to ground level for small load currents.
Typical Applications
(V
+
= 5.0 V
DC
)
Figure 6. Basic Comparator Figure 7. Driving CMOS
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